Welding power sources often include a first stage that converts an AC input signal to a DC signal, and a final regulated output stage that converts the DC signal into a signal for welding. The term “welding” includes “plasma cutting”, wherein it is desirable to isolate the welding or cutting process from the input power. Vogel U.S. Pat. No. 5,991,180 discusses a chopper having an output isolation transformer located after welding regulation and directly driving the welding operation, wherein the chopper network creates a desired regulated output welding current and isolation is provided in the output stage. Thommes U.S. Pat. No. 5,601,741 discloses a boost converter driving a pulse width modulated (PWM) inverter that creates a regulated welding output signal, where the second stages of both Vogel and Thommes are regulated to supply the power factor controlled current from a preregulator directly into a welding operation. Welding power sources are shown in Moriguchi U.S. Pat. No. 5,926,381, Moriguchi U.S. Pat. No. 6,278,080, and Moriguchi U.S. Pat. No. 6,069,811 in which a regulated output inverter is driven by an input boost converter or a DC output of a rectifier to produce a current suitable for welding to an output transformer used for isolation, where the output of the transformer secondary is used for the welding operation. There is no three stage topology in the above patents as is used in the novel power source for practicing the present invention. Daniel, U.S. patent application Ser. No. 10/889,866, is assigned to the assignee of the present invention and describes a three stage power source architecture for welding, in which a first stage converts AC power to a first DC output signal, a second stage converts the first DC output signal into a second DC output signal, and a third stage converts the second DC output signal into a process output for welding, where the second stage is unregulated. The Daniel patent application is incorporated herein by reference as background information and is not prior art. The three stage welder of Daniel has a regulated first stage, as is common, and a welding regulated output stage where a welding signal is determined by feedback from the actual welding process. This is also common, but a novel feature of Daniel is an isolation unregulated intermediate stage between the regulated first stage and the output stage, where the output stage is regulated by feedback to create a signal suitable for welding.
With respect to background technology, Boylan U.S. Pat. No. 6,618,274 illustrates a synchronous rectifier, and Calkin U.S. Pat. No. 3,737,755 discloses a DC/DC converter for low power use where a fixed regulated current is directed to a non-regulated inverter to provide a non variable output DC signal. The general background technology in Boylan U.S. Pat. No. 6,618,274 and Calkin U.S. Pat. No. 3,737,755 is incorporated by reference herein to show a synchronous rectifier where any output regulation is performed before the inverter by controlling the level of the input DC signal, where neither of these patents relate to a power source for welding and are only incorporated by reference as general technical concepts, such as synchronous rectifier devices and unregulated inverters. Smolenski U.S. Pat. No. 5,019,952 shows a non-welding two stage AC to DC converter for imparting minimum harmonic distortion to the current flowing into the converter. Unlike welding situations, the load in Smolenski U.S. Pat. No. 5,019,952 is not variable and does not require regulation, wherein this patent is incorporated by reference to show general technology as background information with respect to the present invention.
Switching converters are often employed as the final output stage for creating the output welding current according to a desired welding waveform, where the weld process may require DC or AC current waveforms to create a welding arc between an advancing electrode and the workpiece being welded. Such converters are typically PWM designs, in which switches are operated at high frequency to create the desired waveform or current level for the welding process, for example, as discussed in Blankenship U.S. Pat. No. 5,278,390. In modern arc welders, the final converter stage often employs “waveform control technology” pioneered by The Lincoln Electric Company of Cleveland, Ohio where the welder output is generated using a series of short pulses at a frequency generally above audible levels and the group of short pulses has a waveform or profile controlled by a waveform generator. As shown in Kooken U.S. Pat. No. 5,991,169 and Church U.S. Pat. No. 6,504,132, the welding output current can be regulated by an output chopper or buck converter, with isolation being achieved using a transformer either in the output of an inverter stage or in the output of an input boost converter.
Switching converters, such as buck, boost, or other type DC to DC converters, have been developed in non-welding contexts, which include two or more converter phases or cells for inputting DC power and providing a DC output. Such converters are sometimes referred to as multiphase converters, for example, as shown in Fletcher U.S. Pat. No. 3,984,799 and Ogawa U.S. Pat. No. 4,748,397. Huang, “A Scalable Multiphase Buck Converter with Average Current Share Bus” and Schuellein, “Multiphase Converter Bucks Power” describe scalable multiphase converters targeting advanced microprocessor applications. Cho “Novel Zero-Voltage-Transition PWM Multiphase Converters” illustrates two and three-phase DC to DC converters with a single auxiliary zero-voltage switching (ZVS) circuit to reduce switching losses. Multiphase converters have also been employed in automotive applications, as discussed in Karadsheh U.S. Pat. No. 4,433,370 and Czogalla “Automotive Application of Multi-Phase Coupled-Inductor DC-DC Converter”, where Czogalla discusses coupling inductors of individual phases together on a common core. Coupled inductors in multiphase interleaved regulator modules and converters are also described in Wong “Performance Improvements of Interleaving VRMs with Coupling Inductors”; Zumel “Magnetic Integration for Interleaved Converters”; and Dixon “Coupled Filter Inductors in Multi-Output Buck Regulators”. These references are incorporated by reference herein as background information, and do not teach use of multiphase converters in a three stage power source. Baker U.S. Pat. No. 5,864,116, shows a two-phase down chopper with coupled inductors for welding, and is assigned to the assignee of the present invention. Reynolds U.S. Pat. No. 6,051,804 and Reynolds U.S. Pat. No. 6,300,589 illustrate a plasma cutting power supply having dual choppers providing power from a voltage source to a load, in which the open circuit output voltage is approximately twice the load output voltage. However, neither Baker nor the Reynolds patents teach multiphase output stages in a three stage welding power source.
In welding systems, the power efficiency of a welding power source is an important design parameter, where low efficiency power sources produce excess heat, and are generally larger and more bulky than more efficient sources. In general, it is desirable to reduce or minimize the electrical switching and conduction losses in the components of a welder power source to increase the efficiency. Furthermore, it is desirable to minimize ripple currents in a power source to minimize electrical stress to capacitors and other components, as well as to improve the quality of the weld operation. Another design goal is fast transient or impulse response (e.g., high slew rate), wherein it is desirable to provide a welder power source able to transition quickly between different output signal levels for waveform control and to quickly adapt to changing load conditions, particularly for short-circuit welding and other applications in which welding arc conditions may change rapidly. In this regard, welding power sources typically have very different operational requirements than most power supply designs in which load fluctuations are minimal. In addition, welding power sources often include large filter capacitors and/or series inductors or chokes to maintain output signal levels and internal DC voltages within certain ranges or limits during fast load transients, wherein the need for such filtering or smoothing components is greater if the switching converter controls are bandwidth limited.
In the advancement of welding power sources, it is therefore desirable to increase the operating bandwidth of the final output stage to mitigate or avoid the need for large filtering components and to thereby improve transient response of the source. Although less filtering facilitates improved slew rates, reduced output filtering may lead to higher ripple currents and voltages. Furthermore, switching losses generally increase as the switching converter operating frequency is increased. Simply increasing the switching speed of an output chopper stage would require larger switching devices to withstand the additional heat generated and/or additional or larger heat removal devices, such as fans, heat sinks, etc., whereby the component count, size, and cost of the welding system increases and the system power efficiency is worsened. One possible approach is to increase the converter bandwidth or switching frequency while employing so-called soft-switching techniques to power transistors and other components in the output chopper stage to reduce the amount of switching losses, and also to potentially reduce the amount of electromagnetic or radio frequency interference (EMI, RFI). However, using soft switching requires additional auxiliary components, reduces chopper efficiency, and the auxiliary inductors and rectifiers are subjected to high currents. Thus, there is a need for improved welder power sources with higher bandwidth switching converter output stages, by which good transient response can be achieved without significantly impacting system cost and efficiency.